Generally, it is known that a compound having a perovskite structure and comprising Y-including rare earth element (hereinafter, indicated by a symbol R), an alkaline earth metal (hereinafter, indicated by a symbol A), copper (Cu) and oxygen (O) (hereinafter, the compound being referred to as a superconducting ceramic) exhibits superconductive phenomena at a temperature of 77.degree. K., which is a temperature that can be achieved by liquid nitrogen.
For fabricating a superconducting ceramics wire using the above-described superconducting ceramics powder, there is known a process which includes the following steps:
providing starting powders, i.e., an R.sub.2 O.sub.3 powder, an alkaline earth metal carbonate powder as the component A, and CuO powder, each having an average grain size of not greater than 10 .mu.m, compounding and mixing them in a predetermined compounding ratio, to obtain a mixed powder, calcining the mixed powder in the air or in an oxygenic atmosphere, at a temperature of from 850.degree. to 950.degree. C. to form a superconducting ceramics material having a perovskite structure, and grinding the ceramics to obtain powder of an average grain size of not greater than 10 .mu.m,
filling a pipe of silver (Ag) with the superconducting powder obtained in the previous step, sealing the both ends of the pipe under vacuum, and subjecting the silver pipe filled with the ground powder to cold-drawing, e.g., swaging, rolling with grooved rolls, processing with a die, or the like, to produce a wire having a diameter of not greater than 5 mm,
heat-treating the wire in the air or an oxygenic atmosphere at sintering temperatures (for example, in the case of using Y-based superconducting ceramics oxide, the temperatures being within from 900.degree. to 950.degree. C.) to produce a superconducting wire.
Also, a superconducting cable has been conventionally produced by bundling a plurality of the wires produced by cold-drawing as described above, covering the bundled wires with a tube of Ag to form a cable, and then subjecting the cable to processing with a die, if required.
However, although the superconducting wires and cables produced by the above-described conventional processes each have a critical current density, Jc, of order of 10.sup.3 A/cm.sup.2, the critical current density, Jc, of superconducting wires and cables for practical use each must be at least 10.sup.4 A/cm.sup.2.
A process for satisfying the requirement comprises the steps of: preparing a monocrystal of a superconducting ceramics having a perovskite structure, and fabricating a superconducting wire using the monocrystal in which an electric current sends therethrough perpendicularly to the C-axial direction of the monocrystal, and a superconducting wire having a critical current density of greater than 10.sup.4 A/cm.sup.2 can be produced (it is generally said that the above-described superconducting ceramics has high anisotropy in the crystal, and then an electric current hardly flows in the C-axial direction, but easily flows in the direction perpendicular to the C-axial direction, and the conductivity of the electric current in the C-axial direction is not greater than about 1/100 of that of one electric current in the direction perpendicular to the C-axial direction.). Even when it would be possible to produce experimentally such superconducting ceramics monocrystal in the form of a long wire, it is impossible to produce it on an industrial scale.
Also, in the case of the superconducting cable, although it would be possible to produce a cable by bundling wires, each of which is produced by filling an Ag sheath with one or more superconducting monocrystal fibers that has each a perovskite structure, and each having a C-axis of the monocrystal being oriented in the direction perpendicular to the longitudinal direction of the fiber, it is difficult to produce such monocrystal fibers at present, and therefore it is also impossible to produce a superconducting cable, comprising a bundle of the wires filled with the monocrystal fibers.